1. Field of the Invention
This invention relates to the separation of a glass panel and a glass body of a cathode ray tube, for example a cathode ray tube intended to act as a picture tube of a television receiver.
2. Description of the Prior Art
Cathode ray tubes (CRTs) intended to act as picture tubes of color television receivers conventionally are constructed by various different manufacturers in a manner that will now be described with reference to FIG. 1 of the accompanying drawings. The CRT is formed from a funnel-like glass body 10, referred to hereinafter more simply as a "funnel", and a glass face plate or panel 12. A metal electrode member 14, known as an anode button, extends through the wall of a main body portion 18 of the funnel 10 and is sealed in a vacuum-tight manner to the glass making up the wall.
To make the CRT, a carbon (graphite) coating (not shown) is applied to the interior of the funnel 10. The composition of the internal graphite coating may vary as between the main body portion 18 and a neck portion 20 which is designed to accommodate an electron gun (not shown). The funnel 10 then is heated in an oven so that the internal carbon coating will cure or set.
The funnel 10 then is joined to the panel 12, the interior of the panel previously having been coated in respective local areas with one or more phosphors, its own carbon (graphite) coating and, in some instances, aluminum. The funnel 10 is joined to the panel 12 by applying an adhesive to an end edge 22 of the funnel and/or to an edge 24 of the panel 12 so that the edges 22 and 24 are joined together by the adhesive. The adhesive comprises a frit and a binder liquid and, after it has been applied, the assembly of the funnel 10 and panel 12 is heated so that the adhesive will cure or set to firmly bond together the funnel 10 and panel 12 in such a manner as to provide a vacuum-tight seal between them.
The binder liquid typically comprises a binding agent (such as nitrocellulose) which is dissolved in a suitable solvent (such as isoamyl acetate). By way of example, though it should be appreciated that the numerical values quoted below may be varied in accordance with various relevant factors, the binder liquid may be formed by adding nitrocellulose to isoamyl acetate in the proportion of 12.6 g of nitrocellulose per liter of isoamyl acetate, and the adhesive may be formed by mixing the frit (powder form) and binder liquid in the proportions of 900 ml of the binder liquid per kilogram of frit powder.
Thereafter, the funnel 10 is evacuated and, whilst it is evacuated, the electron gun (not shown) is sealingly fitted into the neck portion 20 of the funnel and the flared end part of the neck portion is removed. Also, a carbon (graphite) coating (not shown) is applied to at least part of the exterior of the main body portion 18 of the funnel 10. The exterior graphite coating may be of a different composition to that or those on the interior of the funnel 10.
It sometimes happens that, for one reason or another, a CRT manufactured by the method just described will be found to be defective. For example, the seal formed by the adhesive joining together the funnel 10 and panel 12 may be defective so that a vacuum cannot be maintained in the CRT. Instead of throwing away the tube, many manufacturers will, in this event, try to salvage the funnel 10 and/or the panel 12 by separating them, whereupon the various coatings can be removed from the salvaged component and the component can be reintroduced into the manufacturing process at a suitable stage and used in making another CRT.
A process employed by several manufacturers to separate the funnel 10 and panel 12 is as follows. A batch of defective CRTs are oriented in a separation apparatus as shown in FIG. 1 and an aqueous solution of nitric acid (HNO.sub.3) is poured on to the center of the panel 12 of each CRT so that it trickles down past the hardened adhesive joining the panel to the funnel 10. The nitric acid solution penetrates the adhesive to some extent and weakens it by attacking the binder that holds the frit together. However, the use of nitric acid solution alone is not sufficient to destroy the bond between the funnel 10 and panel 12, or at least is not sufficient to destroy the bond in a period of time which is not so long as to make the separation process uneconomical. To destroy the bond, the step of applying a nitric acid solution to the adhesive is followed by the application of a thermal shock. To provide the thermal shock, the flow of nitric acid solution is stopped and replaced by alternating flows of cold water and hot water. The alternate supply of hot and cold water provides differential thermal expansion which destroys the bond between the funnel 10 and the panel 12, which bond previously was weakened by the nitric acid solution, so that they can be separated.
The separation process described above has several disadvantages. Firstly, it is very time-consuming, typically taking about 20 minutes. Secondly, it is not very reliable. Thus, it is sometimes found for a particular CRT that the bond has not been destroyed, that is that the funnel 10 and panel 12 have not become separated, so that the CRT concerned has to be put back in the separation apparatus for at least one further period of around 20 minutes. Thirdly, the thermal shock treatment sometimes stresses the glass funnel 10 and/or the glass panel 12 to such an extent that it will crack and therefore cannot be salvaged. These various disadvantages can have such a cumulative effect that the economic justification for carrying out the salvage operation may become questionable.